Such a differential output amplifier arrangement is already known from standard operational amplifier theory, such as for instance described in the tutorial handbook "Microelectronics: digital and analog circuits and systems", by J. Millman--ISBN nr 0-07-066410-2, Ed 1983. Therein, on page 575, in FIG. 16.8, an improved instrumentation amplifier is shown whereby the first stage consisting of two operational amplifiers A1 and A2, the inverting input terminals of which are coupled via a coupling resistor R, and each amplifier having a feedback resistor R', is a differential output amplifier, as is also mentioned on the same page of the prior art document. This differential output amplifier corresponds to a differential output amplifier arrangement including a first arrangement input terminal and a second arrangement input terminal for respectively coupling to a first terminal and a second terminal of an input signal source, the differential output amplifier arrangement further including a first arrangement output terminal and a second arrangement output terminal for delivering a differential output voltage between the first arrangement output terminal and the second arrangement output terminal, the differential output amplifier arrangement further including a first operational amplifier having a first amplifier input terminal of a first polarity type, and coupled to the first arrangement input terminal, the first operational amplifier having a second amplifier input terminal of a second polarity type opposite to the first polarity type, and an amplifier output terminal coupled to the first arrangement output terminal, a second operational amplifier having a first input terminal of the first polarity type, and coupled to the second arrangement input terminal of the second polarity type, coupled via a first resistor with the second amplifier input terminal of the first operational amplifier, the second operational amplifier further including an output terminal coupled to the second arrangement output terminal, the amplifier output terminal of the first operational amplifier being coupled to the second amplifier input terminal of the first operational amplifier via a second resistor, the output terminal of the second operational amplifier being coupled to the second input terminal of the second operational amplifier via third resistor. Indeed, the respective prior art amplifiers A1 and A2 correspond to the respective first and second operational amplifiers OA1 and OA2, the prior art coupling resistor R corresponds to the first resistor R1, and the two feedback resistors denoted R' in the prior art document correspond with the second and third resistors, as described above.
Such a differential output amplifier has the advantage of having a very high input resistance, as well as an excellent common mode rejection ratio.
When using such a differential output amplifier for delivering maximum power to a load, via transmission lines coupled to the differential output terminals of this amplifier, it is essential to match both the impedance of the load as well as the output impedance of this amplifier, to the line characteristic impedance. The classical solution for this matching consists of artificially increasing the output impedance of the differential output amplifier, by adding a resistor in series with each differential output terminal. For a characteristic impedance value of RL/2, the load series resistor in this differential output amplifier has to be equal to RL, whereas for each of the two differential output terminals, a series resistor with a resistance value of approximately RL/2, is to be coupled in between this terminal and the transmission line.
The resulting configuration however results in an increased power consumption, both in the added series resistors, as well as in both operational amplifiers themselves. This increased power consumption has a detrimental effect on the failure rate of the components, and thus also on the lifetime of the whole arrangement.